1. Field of the Invention
This invention relates to flywheels adapted for the storage of energy and more particularly to such flywheels having a composite construction.
2. Description of the Prior Art
Recently, there has been a revival in the engineering and scientific interest in flywheels. This owes to the fact that flywheels provide an efficient means of energy storage with no adverse environmental impact. Flywheels may be useful as means for energy storage in solar energy systems, mechanical power systems, and electrical power systems. For example, electrical utilities may employ flywheels as a means of storing energy required for times of peak loading. Flywheels may also be useful as means for storing energy for propulsion and auxiliary power in air, land, sea and space vehicles.
Flywheels function by storing kinetic energy. The amount of energy capable of being stored in a particular flywheel is a function of the mass of the flywheel, the distribution of mass within the flywheel, and the maximum allowable speed of rotation of the flywheel. However, the maximum allowable speed of rotation of a flywheel is limited by the strength of the material from which the flywheel is formed. That is, as the rotational speed of a flywheel increases, the internal stresses within the flywheel also increase, which stresses, if allowed to exceed certain limits, would cause the flywheel to break apart or fracture. It is apparent that optimally, flywheels should be constructed from materials having high strength-to-weight ratios.
Hereinbefore, prior art flywheels for the most part have proven unsatisfactory in meeting energy storing requirements within certain constraints of mass and volume. For example, many prior art flywheels comprise discs or solid cylindrical members formed from a homogeneous metal and rotatable about the central axes thereof. Although these homogeneous metal flywheels are formed from high strength materials, the strength-to-weight ratios available in metals and the fracture mechanics of metals under cyclic fatigue conditions severely limit the energy storage capability of the flywheel. Therefore, these prior art metal flywheels tend to be quite heavy. Moreover, should such prior art metal flywheel rupture, pieces breaking off the ruptured flywheel would possess sufficient energy to seriously damage equipment or injure persons in the vicinity. Such prior art metal flywheels are primarily useful where there are no size or weight constraints and where precautions have been taken ot insure the safety of persons or machinery in the area of the flywheel.
To overcome these disadvantages associated with prior art metallic flywheels, composite flywheels were developed. These composite flywheels are normally fabricated from a multiplicity of glass or similar fibers disposed in a matrix or binder of epoxy or any other suitable resin. Such composite flywheels are generally of a high strength-to-density ratio and therefore, to be able to store sufficient amounts of energy, may be required to rotate at extremely high speeds such as tens of thousands of revolutions per minute. These prior art composite flywheels have for the most part been formed by a circumferential distribtution of the fibers within the binder or matrix in the form of flat cylinders or spoke-mounted rings. Since, when a body rotates it is subject to stresses due to centrifugal force in a radially outward direction, such prior art composite flywheels must carry a significant portion of this centrifugal stress in directions normal to the axes of the circumferential fibers from which it is formed, placing the resin matrix in tension. Epoxy, elastomers, and other resins employed with composite flywheels are relatively weak when loaded in tension as compared to the strengths of the fibers. Therefore, a high rotational speed of such a composite flywheel can cause the flywheel to break apart along circumferential lines between the fibers from which it is formed.
Normally a flywheel must be apertured at a central portion thereof to accommodate bolts or other members for mounting the flywheel to a hub or shaft. Such apertures introduce stress concentrations in areas of the flywheel immediately adjacent thereto, substantially weakening the flywheel at these locations. Moreover, due to the mass and weight of the flywheel itself, the centrifugal loading of a prior art flywheel is most severe in these central portions, thereby limiting the rotational speed capability and thus the energy storage capability of the flywheel.
Therefore, it is an object of the present invention to provide a flywheel configuration suitable for use with either composite or isotropic materials wherein centrifugal loading is primarily carried along circumferential directions, reducing levels of stress and strain in the central portion of the flywheel and permitting the direct mounting of the flywheel to shafts or hubs without the rotational speed capability of the flywheel being limited by conditions at the area.
It is another object of the present invention to provide a composite flywheel capable of storing requisite amounts of energy within constraints of limited weight and volume.
It is another object of the present invention to provide a composite flywheel wherein the risk of radial delamination or fracture under high rotational speeds is minimized.
It is another object of the present invention to provide a composite flywheel in which the centrifugal forces are carried by high strength fibers without imposing high loading upon matrix material in which the fibers are located.